Heart failure is the major cause of cardiovascular mortality in the US and often develops as a consequence of maladaptive cardiac remodeling due to hypertension or a myocardial infarction (MI). Although over the past decade we have made major advances in the treatment of heart failure, the morbidity and mortality associated with this disease remains high. Thus, the identification of candidate proteins and signaling pathways that prevent or reverse the process of maladaptive cardiac remodeling is a major goal in the pursuit of new treatment approaches for improving the quality and longevity of heart failure patients. Our laboratory recently identified cytoglobin (Cygb) as a stress-responsive hemoprotein and its levels increase in the adult heart under a variety of stress conditions (i.e. hypoxia, ischemia, and pressure-overload). Overexpression of Cygb in cultured myocytes protects them from oxidative stress, while knockdown of Cygb results in increased apoptotic cell death. Initially, we postulated that Cygb protects myocytes via anti-oxidative mechanisms inherent to all hemoproteins; however, we now have extensive evidence that Cygb is also directly involved in regulating the transcriptional activity of the tumor suppressor protein p53. W have found a reciprocal relationship between transcript levels of Cygb and a number of p53-target genes. Chromatin immunoprecipitation experiments demonstrate co-occupancy of p53 and Cygb at classic p53 target promoters. Using recombinant proteins we show a direct protein-protein interaction between the two proteins. Importantly, recombinant Cygb inhibits p53 transcriptional activity in vitro reconstituted transcriptional assays. Finally, we have developed conditional Cygb knockout mouse model and have preliminary studies demonstrating that acute cardiac-specific loss of Cygb leads to a dilated cardiomyopathy. Likewise, mice born with cardiac-specific deletion of Cygb develop a more pronounced dilated cardiomyopathy after ischemia-reperfusion (I/R) injury as compared to injured control mice. Our central hypothesis is that Cygb contributes to cardiomyocyte survival via both anti-oxidative and p53-dependent mechanisms. We propose the following three specific aims to test this hypothesis: Specific Aim 1: Define the role of cytoglobin in maintaining cardiomyocyte homeostasis. Specific Aim 2: Define the role of cytoglobin during maladaptive cardiac remodeling. Specific Aim 3: Define the significance of the cytoglobin-p53 interaction for cardiomyocyte survival. The successful completion of our proposed study will provide insight into a novel signaling pathway that regulates cardiomyocyte survival and may provide opportunities for the development of novel therapeutic approaches to prevent and/or reverse heart failure. Thus, the proposed NIH R01 Research Grant Application is relevant to and in keeping with the missions of both the NIH and NHBLI.